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IGCSE · Practice questions · Classification and dichotomous keys

Split it in two, then split again.

Six original Cambridge-style questions on classifying by shared features, species and the binomial system, building and using dichotomous keys, and comparing DNA base sequences.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Fertile offspring, two names, two choices.

01
Recall
[2 marks]

The domestic cat is Felis catus and the lion is Panthera leo. (a) State the genus of the lion. (b) Explain why the binomial system is useful to scientists.

  • (a) Panthera. ✓
  • (b) It is internationally agreed, so every scientist uses the same name for a species, whatever the common name in their own language. ✓
02
Explain
[3 marks]

A horse and a donkey can mate to produce a mule. Mules are almost always sterile. Explain whether horses and donkeys belong to the same species.

  • A species is a group of organisms that can reproduce to produce fertile offspring. ✓
  • The mule is offspring, but it is sterile, so it cannot reproduce. ✓
  • So horses and donkeys are different species (they are in the same genus, Equus). ✓
03
Analysis
[4 marks]

Four arthropods have these features. A: three pairs of legs, wings. B: four pairs of legs, no antennae. C: five pairs of legs, two pairs of antennae. D: three pairs of legs, no wings. Construct a dichotomous key to identify A, B, C and D.

  • 1 a: three pairs of legs .... go to 2. 1 b: more than three pairs of legs .... go to 3. ✓
  • 2 a: has wings .... A. 2 b: has no wings .... D. ✓
  • 3 a: has antennae .... C. 3 b: has no antennae .... B. ✓
  • Three pairs of statements for four organisms; each pair is about one visible feature. ✓
04
Explain
[2 marks]

A student's key contains the statements “1 a: is large” and “1 b: is small”. Explain why these are poor statements and suggest a better kind of statement.

  • Size varies between individuals of the same species and with age, and large or small depends on opinion, so an organism might fit either. ✓
  • Use a pair of opposite statements about a clear visible feature, for example “has wings” / “has no wings”. ✓
05
Data
[3 marks]

Extended. Part of the same gene was compared in four mammals. The percentage of bases that are the same as in humans is: chimpanzee 90%, gorilla 85%, mouse 70%. Explain what these results show about the evolutionary relationships of these animals.

  • The chimpanzee sequence is the most similar to the human sequence, so humans and chimpanzees share the most recent common ancestor. ✓
  • The gorilla is the next most closely related, and the mouse is the most distantly related. ✓
  • Mutations build up over time, so species that separated longer ago have more differences in their base sequences. ✓
06
Explain
[2 marks]

Extended. Whales look like fish, but they are classified as mammals. Explain how DNA evidence supports this classification.

  • Classification aims to reflect evolutionary relationships. ✓
  • The base sequences in whale DNA are more similar to those of other mammals than to those of fish, showing that whales share a more recent common ancestor with mammals. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.